All of the songs you have played so far involved a downward pick motion, indicated by a. With the introduction of eighth notes, down (a) and up (s) picking is used. This is called alternate picking. In alternate picking, use a down pick ON the beat (the number count) and an up pick OFF the beat (the "and" count). Try the following exercise:
Use alternate picking on the second beat of the first bar. To achieve the rest on the first beat of bar 2, stop the third string from sounding by lightly touching it with your second finger.
Strum Pattern